1. Field of the Invention
This invention relates generally to electrochemical fuel cell stacks. More specifically, the present invention relates to electrochemical fuel cell stacks comprising a plurality of membrane electrode assemblies stacked in an alternating manner such that the polarity of adjacent membrane electrode assemblies are opposite.
2. Description of the Related Art
Fuel cell systems are currently being developed for use as power supplies in a wide variety of applications, such as automobiles and stationary power plants. Such systems offer the promise of economically delivering power while providing environmental and other benefits.
Fuel cells convert reactants, namely, fuel and oxidant, to generate electric power and reaction products. Fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. A catalyst typically induces the desired electrochemical reactions at the electrodes. Fuel cell types include, for example, alkaline fuel cells and solid polymer electrolyte (SPE) fuel cells, which comprise a solid polymer electrolyte and operate at relatively low temperatures.
SPE fuel cells employ a membrane electrode assembly (MEA), which comprises the solid polymer electrolyte or ion-exchange membrane disposed between the two electrodes. Each electrode contains a catalyst layer, comprising an appropriate catalyst for facilitating the desired electrochemical reaction of the fuel and oxidant, located adjacent to the solid polymer electrolyte membrane. The catalyst is typically a precious metal composition (e.g., platinum metal black or an alloy thereof) and may be provided on a suitable support (e.g., fine platinum particles supported on a carbon black support). The catalyst layers may contain an ionomer similar to that used for the solid polymer electrolyte membrane (e.g., Nafion®). The electrodes may also contain a porous, electrically conductive substrate that may be employed for purposes of mechanical support, electrical conduction, and/or reactant distribution, thus serving as a fluid diffusion or distribution layer.
In electrochemical fuel cells, the MEA is typically interposed between two substantially fluid impermeable separator plates (anode and cathode plates). The plates typically act as current collectors and provide support to the MEA. In addition, the plates may have reactant channels formed therein and act as flow field plates providing access of the fuel and oxidant to the porous anode and cathode substrates, respectively, and providing for the removal of product water formed during operation of the cells. Typically, a plurality of inlet ports, supply manifolds, exhaust manifolds and outlet ports direct the fuel and oxidant to, and the exhaust products from, the reactant channels in these flow field plates. The supply and exhaust manifolds may be internal manifolds, which extend through aligned openings formed in the flow field plates and MEAs, or may comprise external or edge manifolds, attached to the edges of the flow field plates.
A broad range of reactants can be used in SPE fuel cells. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be, for example, substantially pure oxygen or a dilute oxygen stream such as air.
During normal operation of a SPE fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The electrons travel through an external circuit providing useable power and then react with the protons and oxidant at the cathode catalyst to generate water reaction product.
In operation, the output voltage of an individual fuel cell under load is generally below one volt. Therefore, in order to provide greater output voltage, multiple cells are usually stacked together (anode to cathode) and are connected in series to create a higher voltage fuel cell stack. End plate assemblies are placed at each end of the stack to hold the stack together and to compress the stack components together, thereby sealing and providing adequate electrical contact between various stack components. Fuel cell stacks can be further connected in series and/or parallel combinations to form larger arrays for delivering higher voltages and/or currents.
Although there have been advances in the field, there remains a need for improved fuel cell stack designs which are both economical and simple to manufacture. The present invention addresses these needs and provides further related advantages.